Corrosion is a chemical reaction that involves the removal of metallic electrons from metals and formation of more stable compounds such as iron oxide (rust), in which the free electrons are usually less numerous. In nature, only rather chemically inactive metals such as gold and platinum are found in their pure or nearly pure form; most others are mined as ores that must be refined to obtain the metal. Corrosion is a process that simply reverses the refining process, returning the metal to its natural state. Corrosion compounds form on the surface of a solid material. If these compounds are hard and impenetrable, and if they adhere well to the parent material, the progress of corrosion is arrested. If these compounds are loose and porous, however, corrosion may proceed swiftly and continuously.
When two different metals are placed together in a solution (electrolyte), one metal will give up ions to the solution more readily than the other. This difference in behavior will bring about a difference in electrical voltage between the two metals. If the metals are in electrical contact with each other, electricity will flow between them and they will corrode—this is the principle of the galvanic cell or battery. Though useful in a battery, this reaction causes problems in a structure. For example, steel bolts in an aluminum framework may, in the presence of rain or fog, form multiple galvanic cells at the point of contact between the two metals, corroding the aluminum.
Corrosion testing is performed to ascertain the performance of metals and other materials in the presence of various electrolytes. Testing may involve total immersion, as would be encountered in seawater, or exposure to salt fog, as is encountered in chemical industry processing operations or near the oceans where seawater may occur in fogs. Materials are generally immersed in a 5 percent or 20 percent solution of sodium chloride or calcium chloride in water, or the solution may be sprayed into a chamber where the specimens are freely suspended. In suspension testing, care is taken to prevent condensate from dripping from one specimen onto another. The specimens are exposed to the hostile environment for some time, then removed and examined for visible evidence of corrosion. In many cases, mechanical tests after corrosion exposure are performed quantitatively to ascertain mechanical degradation of the material. In other tests, materials are stressed while in the corrosive environment. Still other test procedures have been developed to measure corrosion of metals.
Various testing methods have been utilized in the field of protection against corrosion.
A common method of corrosion measurement is by the method of measuring the loss in mass of the metallic specimen as it is immersed in the corrosive fluid medium. At precise periodic intervals, the test specimen is removed from the fluid medium and subsequently weighed to determine its instantaneous mass. It is then returned to the corrosive bath where the test is resumed. This method of determining the corrosion by a loss of mass is subject to errors in measurement if the temperature of the corrosive medium and the temperature of the specimen under test are not held constant, especially when the specimen is removed for the weight measurement. Fluid adhering to the specimen also contributes to the measurement errors.
Another common method of corrosion measurement is by electrical measurement, utilizing a direct current (DC current), where pieces of wire, tubes or disks are inserted into a corrosive medium and external electrical resistance measurements are taken. The reduction in size of the test object, increases the resistance of the specimen and therefore relates directly to the loss of metal by corrosion and/or erosion. One of the disadvantages of this test method is that the metals used for the specimen are quite temperature sensitive to the thermal gradients if the temperature of the corrosive fluid medium is not held constant. In many environments, such as in pipeline systems, the temperature variations are very extreme, which creates a large uncertainty and inaccuracy in measurement. Another disadvantage in this measurement technique derives from the fact that the specimen itself is subject to the galvanic action created between the specimen and the fluid medium; and, this galvanic voltage cannot be separated from the signal voltage.
Still another measurement method is by a change in inductance of the metallic specimen. To implement this procedure, a probe having a metallic core of steel or iron, encompassed by a coil of wire that conducts an alternating current (AC current), creates a magnetic field about the specimen. The measured impedance is subsequently separated into its quadrature components of inductive reactance and inductive resistance. The loss of metal then corresponds to the change in inductance. This method is advantageous in the reduction of errors due to galvanic action, but is disadvantageous because of the effects of temperature on the specimen and on the coil by the surrounding medium.
Examples of such prior art are shown in the examples that follow.
U.S. Pat. No. 5,854,557, granted Dec. 29, 1998, to E. Tiefnig, discloses an improved corrosion measurement system for determining the rate of corrosion of a metallic specimen immersed in a fluid medium. The system is comprised of a highly sensitive excitation and amplification electronic circuitry for registering and displaying the stable and accurate measurement results.
U.S. Pat. No. 5,583,426, granted Dec. 10, 1996, to E. Tiefnig, teaches of a method and apparatus for determining the corrosivity of fluids on a metallic material by means of passing an alternating current through a coil, having a predetermined frequency, amplitude and waveform, and a metallic specimen with identical composition to the metallic material exposed to the fluid. The specimen held within the magnetic field of the coil, sustains a loss of mass due to the exposure to the fluid media, which results in a change of inductance and inductive resistance.
U.S. Pat. No. 5,243,297, granted Sep. 7, 1994, to A. J. Perkins, et al., discloses an electrical resistance corrosion probe incorporating a temperature sensitive resistor that directly measures the temperature of the probe and of its environment as the corrosion measurements are being made. The temperature sensitive resistor has one end connected to the common junction between the test and reference elements of the corrosion probe and has its common line connected in the common line to several corrosion measuring circuits, including the test, reference and check circuits.
U.S. Pat. No. 4,426,618, granted Jan. 17, 1984, to C. Ronchetti, et al., discloses a probe for the continuous in-situ measurement of the rate of corrosion of pipes that are subjected to high temperatures or having high resistivity liquids flowing through.
U.S. Pat. No. 3,934,646, granted Jan. 27, 1976, to R. S. Robertson, et al., discloses a method and apparatus for determining the corrosion rate in the cold end of a boiler system, having a probe loop of an organic solvent circulating through the loop at a predetermined boiling temperature. A removable specimen is periodically tested for acid deposition or corrosion rate. Recirculating the organic solvent maintains the surface temperature of the specimen within close limits, approximating the maximum corrosion temperature.
The prior art recited above does not teach of the novel advantages that are found in the present invention.
It is therefore an object of the present invention to provide an improved corrosion measurement system that is comprised of a novel temperature compensated measurement circuit and a newly designed electrical resistance corrosion probe.
It is another object of the present invention to provide an improved corrosion measurement system that is comprised of a novel temperature compensated measurement circuit that utilizes the reference element within the corrosion probe as a temperature determining element.
It is still another object of the present invention to provide an improved corrosion measurement system that is comprised of a novel temperature compensated measurement circuit that utilizes the reference element within the corrosion probe as a temperature determining element of the surrounding fluid medium.
It is still yet another object of the present invention to provide an improved corrosion measurement system that is comprised of a novel temperature compensated measurement circuit that maintains the voltage across the reference element within the corrosion probe constant.
Yet still another object of the present invention is to provide an improved corrosion measurement system that is comprised of a novel temperature compensated measurement circuit that maintains the voltage across the reference element within the corrosion probe constant by utilizing current feedback to control the corrosion probe's excitation.
An additional object of the present invention to provide an improved corrosion measurement system that utilizes a unitized probe that houses physically matched elements by having the temperature gradient between the reference element and the corroding element, be near zero because of the thermal tracking due to the equality of the thermal inertia of the two masses.
It is a final object of the present invention to provide an improved corrosion measurement system that utilizes a unitized probe that incorporates physically matched elements that are thermally bridged to maintain a constant temperature gradient between the reference and the corroding elements.
These as well as other objects and advantages of the present invention will be better understood and appreciated upon reading the following detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings.